Messaging company, recently valued at roughly $7 billion, aims to debut in the first half

Slack Technologies Inc. is actively preparing for an initial public offering in the first half of 2019, with an eye toward going public as soon as the first quarter, according to people familiar with the company’s plans.

Slack expects it could achieve a valuation well in excess of roughly $7 billion—the level at which a recent round of financing valued the company, these people said. Still, valuations can change until a company prices its IPO.